The invention relates to a method of regulating the counterpressure in an apparatus for plasticising and quantitatively regulating plastic material by means of a screw which is driven by separate servo motors for the rotary movement and the axial movement, wherein the deviation of the counterpressure from the reference value is detected by means of at least one sensor.
In a known method of that kind (see European patent application EP 0 230 488), regulation of the dynamic pressure is effected by varying the axial speed of the screw. As has been set forth in particular in EP 0 480 350, considerable disadvantages are involved if the screw position is not accurately defined as a function of time. More specifically, that makes it difficult to precisely measure the amount of plastic material to be injected.
In EP 0 480 350 the axial movement of the screw is effected by means of a hydraulic drive, in respect of which it is characteristic that the applied pressure is precisely known, whereas the respective position has to be determined by measurement. In that known method, it can therefore be assumed that the dynamic pressure corresponds to the predetermined value whereas the screw position has to be regulated, which is effected indirectly by way of the change in the speed of screw rotation.
The invention is based on an apparatus in which the axial movement of the screw is also produced by means of a servo motor. With a drive of that kind, it can be assumed that the precise position of the driven screw is known at any time. The current strength which occurs in the servo motor is on the other hand only a very indirect measurement in respect of the torque generated by the motor, and even more so for the dynamic pressure. The latter therefore has to be measured by way of sensors. There is therefore the possibility, in accordance with the invention, of improving a method of the kind set forth in the opening part of this specification, in that the axial movement of the screw is predetermined and deviations of the pressure from the reference value are compensated by altering the rotary movement of the screw.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.